1. Field of the Invention
The present invention relates to exhaust gas purifying apparatuses.
2. Discussion of the Background
Conventionally, a diesel particulate filter (DPF) of porous ceramic has been used for collecting particulate matter (PM) primarily of C (carbon) emitted from a diesel engine. With such a diesel particulate filter, there occurs gradual deposition of particulate matter with continual use thereof, and thus, it has been practiced in the art of exhaust gas purifying apparatus that uses a diesel particulate filter to remove the deposited particulate matter by causing a burning process inside the diesel particulate filter periodically and regenerate the diesel particulate filter.
It is preferable that such regeneration of the diesel particulate filter is conducted during the operation of the diesel engine, without replacing or dismounting the filter, and thus, it is practiced in the art to carry out fuel injection in the state that the piston is moving down in the cylinder following combustion to form a high temperature gas (post injection process). Thereby, the deposited particulate matter is burned with the high temperature gas thus formed.
FIG. 1 shows the overall construction of a conventional exhaust gas purifying system of a diesel engine equipped with a diesel particulate filter according to a related art of the present invention.
With the conventional exhaust gas purifying system explained with reference to FIG. 1, it should be noted that such regeneration of filter is conducted each time the vehicle has traveled a predetermined mileage such as 500 km, over the duration of 10 minutes, for example.
In the case the filter regeneration by way of post injection has been conducted impartially, the regeneration is carried out irrespective of actual amount of collection of the particulate matter in the filter. Thus, in order to ensure that there occurs no excessive deposition of the particulate matter in the filter, there is a need to set the interval of filter regeneration to be shorter than what is actually needed for the sake of safety.
On the other hand, there is a known construction of carrying out regeneration of the diesel particulate filter 12B by way of posit injection as shown in FIG. 3, in which a differential pressure ΔP is measured between the upstream side and downstream side of the diesel particulate filter 12B and the posit injection is carried out when the foregoing differential pressure ΔP has reached a predetermined value. Reference should be made to the U.S. Pat. No. 6,952,920.
Further, U.S. Pat. No. 5,651,248 describes the construction that uses, in addition to the diesel particulate filter, a detection filter and evaluates the amount of the particulate matter collected in the detection filter by measuring the electric resistance. According to this technology, the particulate matter collected by the diesel particulate filter and the particulate matter collected by the detection filter are subjected to burning by using a heater when the detected resistance has decreased below a predetermined value. With this, regeneration of filter is achieved.
As shown in FIGS. 5A, 5B, and 5C, there can be a situation in which the thickness of the collected particulate matter changes in spite of the fact that the deposition amount thereof is the same. Now, when the thickness of the collected particulate matter is different, it becomes difficult to measure the electrical resistance precisely, and there tends to be caused error in the evaluation of the deposition amount.
Further, in the case there is caused a deposition of ash in the diesel particulate filter or detection filter after burning of the particulate matter, no precise measurement of electrical resistance is possible anymore and there tends to be caused a large error in the evaluation of the deposition amount.
Further, with the use of the detection filter, there is caused degradation in the filter or electrode with time or with use in the ambient of exhaust gas. Particularly, the electrode (terminal formed of a conductive metal) is formed by infiltrating a metal such as Cu, Cr, Ni, or the like, and thus, there is a tendency of causing problems of physical degradation, oxidation degradation and thermal degradation, such as oxidation, adhesion of impurities, cracking, corrosion, and the like.
When there is caused degradation in the filter or electrode, it is no longer possible to carry out precise measurement of the electric resistance and error is tend to be caused in the evaluation of the deposition amount of the particulate matter.
The contents of U.S. Pat. Nos. 6,952,920 and 5,651,248 are incorporated herein by reference in their entirety.